Offshore mooring of vessels is necessary for a variety of reasons. A common reason is that harbors or ports do not provide sufficient draft for large vessels. Another reason is that the length/beam of such vessels cannot be accommodated in the harbor.
It is frequently necessary to load large cargo vessels by a “lightering” sequence, where one or more smaller vessels bring cargo to the large vessel and sequentially are offloaded to the large vessel. It can be appreciated that sea and weather conditions can impact the ability to carry out this loading procedure.
A representative cargo vessel loading situation will be described, for purposes of this patent application. It is to be understood, however, that the later-described invention may be applied to any of a number of different vessel loading or mooring situations.
By way of example: a large ocean-going vessel, referred to as the “mother ship,” must be loaded with a heavy material, such as iron ore. Prior art practice was for the mother vessel to anchor with its own chain and anchor, with a single point anchor arrangement, at a location offshore a shore facility. At times, the mother ship would moor to some fixed structure such as a dock, piling or the like. From whatever single point mooring, the mother ship would be “weathervane” or swing around to a position or heading determined by a combination of the various environmental forces acting upon the vessel. It can be appreciated that the vessel would then move, including heading, pitch and roll, in response to the environmental forces. As is commonly known, the environmental forces on such a vessel include wind, waves, and current. An intermediate vessel, often called a transfer vessel or trans-shipper, can then be moored to the side of the mother ship.
The iron ore or other bulk material being shipped is brought to the mother ship from a nearby shore facility by first loading the ore from the shore facility to a relatively small vessel, such as a barge. The barge then moors to the trans-shipper, the cargo is moved from the barge to the trans-shipper by a crane and scoop or the like, and a conveyor system mounted on the trans-shipper moves the ore from the trans-shipper to the mother ship. A number of barge loads are generally required to load the mother ship.
The above-described sequence is by way of example only. In other situations, the trans-shipper is loaded at the shore facility, and moves to the mother ship for the loading process, eliminating use of the barges. In still other settings, the trans-shipper is moored via a single point mooring, and the cargo vessel is moored to the trans-shipper.
The environmental forces mentioned above (wind, wave, and current) can cause problems with this loading procedure. Under the typical single-point mooring arrangement, all of the vessels, particularly the trans-shipper and the barge, are exposed to the environmental forces. As such forces increase, the effect on the mother ship, and particularly on the (typically) smaller trans-shippers and barges, can render off-loading difficult and hazardous, and in bad enough weather conditions loading may be necessarily stopped. Movement of the mother ship to various headings, and heave, pitch and roll of the mother ship, are examples of undesirable vessel movement.
It would be advantageous to have a system and means to determine the optimum position, principally heading, at which to maintain the mother ship and/or trans-shipper to optimize safe and efficient loading. Such optimization of mother ship and/or trans-shipper position would minimize mother ship movement (both as to heading and heave/roll/pitch), and would create the optimum “lee” area adjacent the mother ship, to shelter the (typically) smaller vessels from the prevailing environmental forces, and to enable safe vessel loading in a much broader range of weather conditions. Optimization of mooring position also results in reduced motions, particularly roll, thereby reducing relative ship-to-ship motions.